High-octane gasoline blending stocks produced by Fluid Catalytic Cracking (FCC) units and catalytic reformers have been more extensively used for automobile gasoline, since introduction of regulations on use of lead compounds, e.g., tetraethyl lead, as octane improvers. Furthermore, improvement of automobile mileage is increasingly socially required, which calls for higher engine compression ratio and hence higher-octane unleaded gasoline.
Such high-octane, unleaded gasoline contains large proportions of high-octane gasoline component stocks, e.g., those produced by FCC units and reformers, and toluene. For example, Japanese Patent Publication No. 3-21593 discloses unleaded, high-octane gasoline composed of reformate as the heavier fraction and FCC naphtha as the lighter fraction to have a research octane number of 96 or more. Japanese Patent Publication No. 7-10981 discloses unleaded, high-octane gasoline containing, as the essential components, reformate of specific properties, alkylate and isopentane, to have a research octane number of 99.5 or more. Octane number of reformate has been increased by increasing severity (high temperature operation) of reformers, fractionating reformate to extract higher-octane fraction and such like.
It is noted, however, that unleaded, high-octane gasoline causes several problems while it is stored or in service, such as accelerated formation of gums to clog devices associated with tank, and fuel systems (in particular, fuel filters) in the engine. The more functional gasoline engine is more sensitive to the effects of deposits in the air-intake system on engine performance. For example, the electronically controlled fuel injection device precisely controls air/fuel ratio to improve engine performance, and to improve mileage and exhaust gas composition. However, air/fuel ratio will be no longer adequately controlled when deposits are formed on the air-intake valve, because they will work as obstacles to flow of gasoline ejected out of the fuel injection device, with the result that its operability is lowered. Deposits formed on the combustion chamber walls, on the other hand, tend to increase octane requirements. Therefore, there have been strong requirements to control formation of deposits, both in air-intake system and combustion chamber.
A number of techniques have been proposed to reduce gums in gasoline. For example, Japanese Laid-open Patent Application No. 10-77486 discloses gasoline incorporated with an aliphatic nitroxide compound to control formation of gums. Japanese Laid-open Patent Application No. 9-95688 discloses gasoline aimed at improvement of cleanliness in an air-intake valve and port in a gasoline engine, claiming that formation of deposits on combustion chamber walls can be controlled when gasoline has an octane number of 98 or more, 50% distillation point of 75.degree. C. to 95.degree. C., 97% distillation point of 155.degree. C. or less, aromatic hydrocarbon content of 35 vol % or less, and content of 10 vol % or less for aromatic hydrocarbons having a carbon number of 8 or more. Japanese Laid-open Patent Application No. 9-286992 discloses that an unleaded gasoline composition shows excellent effects of cleaning an air-intake system and combustion chamber, when it is incorporated with a polyetheramine-based detergent at 70 ppm or more and satisfies a specific relationship involving aromatic hydrocarbon content and distillation temperature.
However, none of these techniques shows sufficient effects of controlling formation of gums, or improving cleanliness in air-intake system or combustion chamber. In particular, the technique which depends on use of an additive tends to increase gasoline production cost.
It is an object of the present invention to provide an unleaded, high-octane gasoline composition which forms little gums, and shows excellent effects of cleaning an air-intake system and combustion chamber of a gasoline engine.